Warm- and Cold-Blooded
Animals

No matter what the outside
temperature may be, your
body, like a living furnace,
works to maintain a constant
internal temperature. It
generates heat by burning
the food you eat. All mammals
and birds are capable of
generating this internal
heat and are classed as
homoiotherms (ho-MOY-ah-therms),
or warm-blooded animals.
Normal temperatures for
mammals range from 97° F
to 104° F. Most birds
have a normal temperature
between 106° F and 109° F.

Larger animals, such as these prairie dogs, do not use as much energy to produce the heat required to keep their larger bodies warm.

A portion of the brain
known as the hypothalamus
(hi-po-THAL-ah-mus) is the
thermostat that controls
your body’s furnace.
This thermostat is set at
98.6° F, but a degree
or so higher or lower is
within the normal range
for a human. In fact, your
body temperature varies
with the time of day. It
is at its lowest just before
you get up in the morning,
rises to a peak in the afternoon,
and then falls again while
you sleep at night. Strenuous
activity raises the body
temperature. Illness also
may cause a greater rise
or drop in the normal temperature.

Nerves in the skin and
deep within the body send
temperature messages to
the hypothalamus. It compares
the temperatures of these
areas with that of the brain
and, if they are too low
or too high, it sends messages
to nerves and glands to
help increase or decrease
the heat. When you are cold,
a message from the brain
causes your muscles to shiver.
This generates a little
heat and starts warming
the body. When you are too
hot, a message triggers
your sweat glands. Evaporation
of the resulting perspiration
cools the skin. Another
message may dilate (enlarge)
the blood vessels under
the skin so more blood can
come to the surface and
more heat can escape through
the skin to the air.

A tiny hummingbird must refuel its body furnace every ten to fifteen minutes during the day to maintain its body heat.

Panting is another cooling
method used by mammals with
few sweat glands. Moisture
evaporates from the mouth
and tongue to cool the overheated
body. Birds cannot sweat,
but they get rid of excess
body heat by breathing it
out. Special air sacs, which
extend from the lungs, increase
the amount of air the birds
can breathe in and out.

Warm-blooded animals
can be as active in winter
as summer, but their bodies
must have plenty of food
to burn for additional
heat. Birds, with their
higher body temperatures,
often find it difficult
to locate enough food
when winter’s lower
temperatures arrive, so
most of them migrate to
warmer climates where
their bodies do not have
to work as hard to maintain
heat.

Cold-blooded animals cannot generate their own body heat, but they do regulate it by changing their environment. Alligators and other reptiles often lie in the sun to warm themselves. On the other hand, they cool off by taking a dip in the water, moving into the sade of a rock or crawling into a burrow in the ground.

Heat escapes from the
body through the skin.
Layers of clothing help
you retain your body heat
in the winter. Other mammals
must rely on layers of
fat or a fur covering
to insulate them from
the cold and retain their
body heat. In extremely
cold climates, you won’t
find mammals with large
ears or long tails. A
lot of extra food would
be required to replace
the heat lost from these
large surfaces—food
that would be extremely
hard to find.

Smaller animals must
produce more heat to keep
warm than larger ones.
To understand this, pretend
that a 3-inch-square box
is a small animal and a
6-inch-square box is a
larger animal. On its six
exposed sides, the small
animal has 54 square inches
of skin. The larger animal
has 216 square inches of
skin, or four times as
much. The inside heat-producing
area of the small animal
is 27 cubic inches, but
the inside of the larger
animal contains 216 cubic
inches, which is eight
times bigger. If it takes
one unit of energy for
each cubic inch to warm
1 square inch of skin,
the smaller animal must
burn twice as much energy
to keep its skin at the
temperature of the large
animal’s skin. This
means it must produce twice
as much heat.

When temperatures drop, cold-blooded animals become less active, even sluggish.

Because small bodies
must produce so much heat
to stay warm, the size
of warm-blooded animals
is limited. If the animal
were too small, it could
not digest food fast enough
to produce heat as quickly
as warmth could be lost
through the skin. During
the day a tiny hummingbird
refuels its furnace with
food every ten to fifteen
minutes. If it were not
able to slow its body
down at night to about
one-twentieth of its daytime
energy by going into a
hibernation-like torpor,
the cool night air of
even a warm climate would
endanger the hummingbird’s
life.

Torpor is a type of sleep
from which an animal cannot
be awakened quickly. Its
body temperature drops
to that of its surroundings,
and the heartbeat and
breathing are slowed down
greatly. If the temperature
drops too low, the animal
will freeze and never
awaken from torpor. True
hibernators pass in and
out of torpor throughout
the winter.
Animals that cannot generate
internal heat are known as
poikilotherms (poy-KIL-ah-therms),
or cold-blooded animals. Insects,
worms, fish, amphibians, and
reptiles fall into this category—all
creatures except mammals and
birds. The term cold-blooded
is a little misleading because
poikilotherms can have very
warm body temperatures in
the tropics. Cold-blooded
actually means the animal’s
body temperature is basically
the same as its surroundings.
A fish swimming in 40° F
water will have a body temperature
very near 40° F. The same
fish in 60° F water will
have a body temperature near
60° F.

After a cool night, a grasshopper may be too stiff and cold to hop until the morning sun warms its body.

Since cold-blooded animals
cannot generate their
own heat, they must regulate
their body temperature
by moving to different
environments. You probably
have seen a lizard, turtle,
or alligator lying around
basking in the sun. It
does this to raise its
body temperature. When
it gets too warm, it moves
into the shade, takes
a dip in the water, or
burrows under a rock or
into the ground to cool
off. When temperatures
drop, cold-blooded animals
become less active, even
sluggish. If an insect
becomes too cold, its
wing muscles cannot move
fast enough for it to
fly. Some moths vibrate
their wing muscles, an
action similar to your
shivering, and the contracting
muscles produce enough
heat for takeoff. After
a cold night, a grasshopper
often is too stiff and
cold to hop. However,
once the sun’s rays
have warmed it up, it
can leap around as usual.

Extreme changes in environmental
temperatures can be fatal
to the cold-blooded animal.
As water temperatures
increase, oxygen content
is reduced. Raising the
temperature from 41° F
to 95° F will cut
the oxygen level in half.
A fish experiencing this
drastic rise in temperature
must pump twice as much
water across its gills
to get the same amount
of oxygen it received
when the temperature was
lower. The increased activity
also increases the fish’s
need for oxygen, which
compounds the problem.
As a result, the fish
may die from a lack of
oxygen, not heat.
Many insects die when
temperatures drop, but
next year’s supply
winters in eggs, cocoons,
or some other protective
covering. They emerge
or hatch when spring or
summer temperatures return.
Reptiles burrow into the
ground or find a den in
which to live until surface
temperatures are more
favorable. In fact, sunny
winter days bring many
of them out to warm themselves
and look for food. Extremes
of heat and cold are hard
on all animals. But both
warm-blooded and cold-blooded
animals have adapted to
normal weather changes.